In a combined cycle generator system, exhaust heat from a first system, referred to as the top cycle, is used to generate power in a second system, referred to as the bottom cycle. Such combined cycle systems typically employ a combustion turbine in the top cycle, and a steam turbine in the bottom cycle. A heat recovery steam generator (HRSG) uses the hot exhaust gas from the combustion turbine to produce steam which drives one or more steam turbines.
Cooling the combustion turbine is critically important. The combustors and transitions of a combustion turbine are exposed to extreme heat and require substantial cooling. For example, the combustion turbine inlet gas which travels through the combustion turbine transition pieces may reach temperatures of 1425.degree. C.
Recent combustor and transition cooling designs employ closed systems in which a coolant circulates within the component, thus allowing an increase in turbine inlet temperature without raising flame temperature. The coolant may comprise steam or air. Where steam is the selected coolant, it is often removed from the steam turbine, and used to cool components in the combustion turbine. After cooling the combustor and transition, the steam is re-routed to the steam turbine where useful energy is recovered.
A prior art two cycle generating system as described above is pictured in FIG. 1. As shown, a combustion turbine 2 is coupled to a heat recovery steam generator (HRSG) 6 via an exhaust duct 4. The HRSG 6 has access to a supply of water which is pumped 16 from a condenser 14 located in the bottom cycle of the two cycle system. The hot gas exhaust exiting the combustion turbine 2 heats the water flowing through the HRSG internal tubing 7 and thereby generates steam. That steam, after being routed through a valve 10 and duct 8 apparatus, powers the steam turbine 12.
A portion of the steam from the high pressure section of the steam turbine 12 is routed via a duct 20 to the combustion turbine 2. The steam enters the cooling channels of the combustors, transitions, and blading. The steam thereby cools the combustion turbine walls and blading by absorbing heat. The steam is then commonly returned via a duct 5 to the steam turbine.
When the steam turbine is operating stably at normal operating speeds and temperatures, obtaining steam for cooling the combustion turbine 2 is easily accomplished. However, during startup, or that time when the system is just beginning to operate, sufficient steam is not available to cool the combustion turbine 2. Typically, the HRSG 6 employed in a two cycle system must be large in order that it be able to generate large quantities of steam to power the steam turbine 12. However, a large HRSG 6 does not react quickly to the heat of the combustion turbine exhaust. The HRSG 6 does not become warm sufficiently quickly to generate steam which can be used in cooling the combustion turbine 2 during startup. Without sufficient steam, the danger exists that components of the combustion turbine 2 could be damaged by excessive heat.
One possible method of providing cooling steam during startup would be to employ a conventional auxiliary steam generator. However, this would prove to be an inefficient solution. If an auxiliary steam generator were employed, a separate source of fuel would be required to operate the auxiliary steam generator. Also, during the periods when the auxiliary steam generator would be in use, the HRSG most probably would remain idle and as a consequence the heat generated by the combustion turbine would not be put to productive use. Furthermore, once the combustion turbine reached a normal operating temperature and the HRSG began to operate, the auxiliary steam generator would no longer be required and most likely would remain idle. Thus, employing a conventional auxiliary steam generator would require excess fuel and would be an inefficient use of resources.
Applicant has recognized that sufficient steam to cool the combustion turbine 2 cannot be generated during startup by the HRSG. The equipment currently used to generate steam is too massive and therefore unreactive at the early stages of system operation. Further, conventional auxiliary steam generators do not provide an efficient solution to the problem.
It is therefore desirable to provide an efficient system which supplements the normal steam generating apparatus so as to provide adequate and efficient cooling to the combustion turbine 2 during startup.